GCC 4.0.1 RC2

[Benjamin Kosnik]

> PR 22111 is about libstdc++-v3 being built with binutils 2.15, while
> 2.15.90 or later are required by the patch. 

I say we solve this instead by enabling the abi checking rule only for
those platforms that are using symbol versioning. In addition, we try
to come up with an autoconf macro that tests the linker for the desired
behavior, and doesn't rely on a version number, which we seem to have
difficulty using as a designator for the required behavior on all
hosts. We know that all primary and secondary gcc targets are ok from
testresults postings, so I consider this a bit academic. 

It is my strong preference to not do macro defines in c++config.h as
per your last patch.

-benjamin